1. Field of the Invention
This invention relates generally to direct access storage devices and, more particularly, to read/write head assemblies for use in such storage devices.
2. Description of the Related Art
In a conventional computer direct access storage device (DASD) having a rotating storage medium, such as a magnetic or magneto-optical disk, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk, for example, can comprise a disk substrate having a surface on which a magnetic material is deposited. The digital data stored on a disk comprises magnetic information that is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes. A read/write assembly produces and detects variations in magnetic orientation of the magnetic material as the disk rotates relative to the head, thereby reading data from, and writing data to, the disk surface.
The surface area of each disk in a DASD may be partitioned into sectors having a short servo track information area followed by a user data area. Each sector can be defined by an imaginary radial line extending from the disk center to the disk outer diameter, or circumference. The servo track information generates a readback signal that is used to position the transducing head across the disk surface. The user data area of a sector contains data tracks in which data is recorded by an end user, or disk drive customer. The servo track information area of a sector typically includes a sector marker, track identification data, and a servo burst pattern, which are recorded at the time of disk manufacture. The transducing head used for reading the servo track data is typically the same head that is used for reading the customer data, but typically a different magnetic head is used for writing the customer data. Such dual-element heads permit optimal exploitation of transducer characteristics for the read and write functions. Increased track densities permit storing more customer data in a DASD with no increase in physical disk size. Such higher track densities have been achieved with transducer advancements such as magneto-resistive heads.
A magneto-resistive (MR) head assembly is a dual-element head that typically includes an MR element for reading user data and servo pattern information, and a different element (typically an inductive element) for writing customer data to the disk. A dual-element assembly having an MR read element and a write element will be referred to as an MR head. The MR element of an MR head exhibits a change in resistance when in the presence of a changing magnetic field. The change in resistance of the MR element is transformed into a voltage signal by passing a constant bias current through the MR element. The MR head generates a fluctuating voltage readback signal as the MR head is passed over the magnetic information recorded on the disk magnetic material. In a DASD using digital demodulation, the fluctuating readback signal is digitized and the digital data values of the sampled readback signal are processed to recover the recorded data.
A disk drive DASD typically includes two signal paths for the head readback signal, comprising a data channel and a servo channel. When the MR head is over a customer data field, the readback signal is processed by the data channel so the system can read and write customer data to and from the disk. When the MR head is over a servo field of the disk, the readback signal is processed by the servo channel to read the servo pattern information that is pre-recorded on the disk at the time of manufacture.
A read/write head assembly is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head assembly from data track to data track and, once over a selected track, to maintain the assembly in a path centered over the selected track. Maintaining the head assembly centered over a track facilitates accurate reading and recording of customer data. With the very high track density of current disk drives, even the smallest head positioning error can potentially cause a loss of customer data.
In the servo information area of a disk sector, the sector marker indicates to the read/write transducing head that servo information immediately follows in the track. The track identification area contains a binary representation of the servo track (or data track) associated with the servo information. The servo burst pattern contains data that generates an analog voltage signal whose magnitude is such that the position of the transducer head relative to a single track can be determined. That is, as the position of the head changes from one edge of a track to the other edge of the track, the magnitude of the readback signal changes, and this is used to keep the head centered in the track for optimal reading and writing of customer data.
A read/write head in a rotary disk drive actuator will exhibit positioning error called track misregistration (TMR) error, which produces errors in reading and writing data to a disk. The total disk TMR is generally made up of two types of error: (1) write-to-read error (WR TMR), comprising the error in reading data from a track into which the data was previously written, and (2) write-to-write error (WW TMR), comprising the error in writing new data into a data track that is adjacent to previously written data. There are various types of disk drive characteristics that can contribute to the total TMR.
For example, some types of error contribute to both WR TMR and WW TMR. These include disk spindle motor runout, which generates an off-center condition to the disk rotation, airflow induced vibration of the read/write head, head seek settling time, electrical and magnetics system noise, and external disturbances such as shock to the drive. These components of total TMR are generally greater toward the outside diameter of the disk (OD), and are lesser toward the inner diameter of the disk (ID). Disk controllers generally include signal processing that is designed to minimize or compensate for such error. Part of such processing may, for example, compensate for the offset between the disk head write element and the disk head read element. These signal processing systems may, themselves, contribute a read-write element compensation error component to the total TMR error, although generally such compensation systems are only involved in read operations subsequent to write operations, and so such errors will only contribute to WR TMR.
Some disk drive design involves minimizing such components of total TMR. For example, FIG. 1 illustrates a conventional MR head assembly in which the read element and the write element are aligned. The servo control system of a disk with such a read/write head will compensate for WR TMR such as described above. Generally, the head configuration of FIG. 1 will result in no tracking error at the inside diameter (ID) of the disk. As the disk head assembly moves toward the outer diameter (OD) of the disk, the servo control system compensates for the skew, or offset, between the tracking of the read element and the write element at the middle of the disk. FIG. 1 is a view looking down on the read/write MR head assembly 100, where the MR head is indicated as having a write element 102 spaced apart from a read element 104, in position above a data track 106 of the disk 108. It can be seen that the write element and read element are aligned along the longitudinal axis 110 of the head assembly, which is also the longitudinal axis of the data track 106 above which the head is positioned. The head assembly 100 has a separation xe2x80x9cdxe2x80x9d between the write element and the read element.
In a different head configuration, the read and write elements of a dual element head are manufactured with a radial offset, such that the heads do not follow along the same longitudinal servo track, but will tend to be aligned in the same servo track only at certain points in the disk arm movement across the disk. Such designs are typically used where it is desired to minimize the read-write element compensation error component of WR TMR. For example, a system that employs a head assembly with radially offset read and write elements is described in U.S. Pat. No. 5,682,274 assigned to International Business Machines (IBM) Corporation.
FIG. 2 shows a schematic plan view representation of a prior art disk drive storage system showing a two-element MR head assembly 200 with a write element 202 and a read element 204 positioned above a track 206 of a disk 208. FIG. 2 illustrates that the write element 202 and read element 204 are radially offset from each other, relative to the longitudinal axis 210 of the MR head. As described in the U.S. Pat. No. 5,682,274, the radial offset of the two elements can be selected so as to minimize the distance necessary to move the MR head for any track on the disk drive to read data following a write operation when the head is midway across the disk. The separation xe2x80x9cdxe2x80x9d between the write element and read element is indicated in FIG. 2, as is the skew angle xe2x80x9ccxe2x80x9d of the read/write element centers relative to the centerline 210. In this way, the processing needed to compensate for read/write element offset is minimized when the head assembly is midway across the disk, and fluctuates generally an equal amount from the ID to OD limits. This is said to make compensation processing less complicated over the entire range of disk arm motion.
Another component of total TMR error not described above is generally referred to as servo track writer (STW) error. This is the error in disk head positioning that can result from using different servo tracks to read and write data from the data tracks. In older disk configurations, a single set of servo tracks could be used for positioning the read/write head over several different data tracks. More recently, the track pitch between data tracks has been steadily shrinking, so that different servo tracks must be used to position the read/write head assembly between read and write operations.
That is, because of increasing data track densities, disk drive systems may exhibit STW errors, in which the read/write head assembly will necessarily use one servo track for positioning the read element and will use a different servo track for positioning the write element, even to read and write data from the same data block on the disk. In that case, each servo track will have an associated servo pattern pitch and runout, or xe2x80x9crulerxe2x80x9d scale, and using the two different tracks for positioning will generate the STW component of total TMR. The STW TMR is added to all other TMR components and contributes to both WR TMR and WW TMR. The STW TMR can increase read/write track misregistration during readback processing of previously written data. Because of manufacturing requirements, most dual element heads cannot be constructed eliminate STW TMR error, and such error can potentially greatly increase, due to increasing track densities.
From the discussion above, it should be apparent that there is a need for a disk drive storage system that more effectively compensates for the STW TMR component of total TMR and increases the accuracy of disk arm positioning with disks of increasing track densities. The present invention solves this need.
The present invention provides a dual element read/write head for a direct access storage device (DASD) wherein the read and write elements are positioned relative to each other such that the same servo track can be used for read and write operations at the disk outside diameter (OD), rather than at the disk inner diameter (ID) or midpoint. As the head is moved across the disk from the OD toward the disk ID, the misalignment between the head elements relative to a data track gets progressively worse, so that different servo tracks are used for positioning the read and write elements as the head moves toward the disk ID. As a result of this head configuration, the contribution of the STW TMR to the total TMR at the OD is minimized to zero for the readback process, so that the total TMR at the OD is primarily a function of only the conventional WW TMR and WR TMR components, including the read-write element compensation error component of WR TMR. Because the misalignment between the read and write elements gets larger as the head assembly moves toward the ID, the total TMR at the ID becomes a function of both the STW TMR and the remaining TMR components. At the ID, however, the read-write element compensation error component of total TMR is smaller than it is at the OD, and can be more easily dealt with, using known servo readback signal processing methods. Therefore, optimizing the read and write element radial offset to minimize that offset at the OD results in a read/write head for which it is easier to carry out the typical signal processing employed by servo controllers. That is, conventional signal processing techniques can be used for positioning the head assembly.
In one aspect of the invention, the radial offset distance x between the write element and the read element is given by an angle xcex1 that is determined by the disk radius distance Rx from the disk center to a tangent point along the OD data track, the distance P from the disk center to the disk arm pivot point, and the distance L from the disk arm pivot point to the tangent point. Thus, the size of the disk and the positioning of the disk arm actuator uniquely determine the radial offset between the write element and read element) and permit control of disk arm movement with conventional servo processing controllers.
Other features and advantages of the present invention should be apparent from the following description of the preferred embodiment, which illustrates, by way of example, the principles of the invention.